Filter septum

Abstract: A filter septum is constructed with a stainless steel sheet microscreen element which has uniformly distributed apertures through which a liquid to be filtered is passed from an inlet side to an exit side of the element. The apertures are preferably tapered from a larger diameter at the inlet side to a smaller diameter at the exit side of the microscreen element. A supporting metal base plate having rows of uniformly distributed holes which are larger then the apertures is brazed to the microscreen element bonding the exit side of the microscreen element to the inlet side of the base plate. Preferably the inlet side of the microscreen element is Teflon® coated.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to filtration systems. More particularly, this invention relates to a filter septum through which liquid to be filtered is passed and the method of manufacturing this filter septum. State of the Art The state of the prior art is accurately set forth in detail in a technical paper entitled "MICROSCREEN FILTRATION OF METALWORKING FLUIDS" presented and published in March 1993 by the Society of Manufacturing Engineers. In this paper the term "microscreen" is defined as a thin metallic sheet with small, uniform apertures or holes throughout. This definition eliminates screens constructed of wire, plastic or synthetic fiber. The advantages of microscreen septums are also presented, centering around higher flux rates made possible by the ability to make the septum aperture smaller, more closely spaced, and more accurately defined by utilizing an electrochemical process in the manufacture of the microscreen. This microscreen is an essential part of the present invention. As also presented in the above-mentioned paper, the difficulty utilizing the advantages of microscreen lies in the fabrication of the filter septum utilizing microscreen. Before the present invention, the most successful use of microscreen has been in placing the microscreen over the outside of a drum of perforated plate by wrapping and clamping the microscreen in position. The difficulties encountered in using only mechanical clamping to secure the microscreen to a perforated substrate or base plate are obvious even in the more successful use in drums and cylinders where often doctor blades are used for scraping and cleaning or pressure swings are used for backflow regeneration. With planar septum configurations the problems can be exasperated. Summary of the Invention The referred to technical paper then presents an improvement to the mechanical clamping fabrication in the form of brazing or "fuse bonding" the microscreen to the base plate support. This innovative bonding fabrication like the electrochemical production of the microscreen is also an essential part of the present invention. The present invention is thus directed to a filter septum according to claim 1 and a method of producing a filter septum according to claim 4. The filter septum of the invention comprises a metal sheet microscreen element having uniformly distributed apertures through which a liquid to be filtered is passed from an inlet side to an exit side of the element. The apertures are formed in the element by an electrochemical process providing a taper in the aperture from a larger diameter at the inlet side to a smaller diameter at the exit side. A metal base plate having rows of uniformly distributed holes which are larger than the apertures is permanently bonded to the microscreen element with a layer of brazing alloy between an inlet side of the base plate and the exit side of the microscreen element. According to the invention, the inlet side of the microscreen element has a non-stick fluorinated ethylene propylene coating. According to the invention parallel support rods are welded to the exit side of the base plate between selected adjacent rows of the holes and the microscreen sheet and the metal base plate can be made of stainless steel, preferably a 316 or 304 stainless steel. The method of producing the filter septum involves the use of a photoetching process to produce the small uniformly distributed apertures in the microscreen element. In a brazing step, an inlet side of the base plate is bonded to the exit side of the microscreen element. If the final form of the filter septum is to be non-planar, for example a cylindrical drum, the base plate will be normally preformed to its final shape prior to brazing. The final step of the process is plastic coating the inlet side of the microscreen element with Teflon®. Brief Description of the Drawings The advantages of the present invention will be more apparent from the following detailed description when considered in connection with the accompanying drawing wherein: FIG. 1 is a perspective view of the filter septum of this invention; FIG. 2 is a plan view of the filter septum showing the exit side of the base plate with its spaced reinforcing rods between adjacent rows of base plate holes, and the base plate is broken away to show the underlying microscreen element; FIG. 3 is an enlarged sectional view taken along line 3-3 of FIG. 2 showing the tapered apertures in the microscreen element and the bonding of the microscreen element to the base plate adjacent one of the base plate holes; FIG. 4 is an enlarged sectional view showing one of the microscreen element apertures and the bonding of the microscreen element to the base plate; and FIG. 5 is an enlarged photomicrograph of the filter septum of this invention. Detailed Description of the Preferred Embodiments of the Invention The filter septum of the present invention can be sized for application to a variety of filtration septums. For example, the star shaped filter 12 described in U.S. Patent 5,221,469 uses a number of panel compartments 40 preferable arranged as shown in diametrically opposed pairs. Referring to FIGS. 5-8 of this patent, each of the compartments has a pair of opposed septums 86 which are typically made of suitable metal or plastic screen material, and in the embodiments shown they are made of stainless steel wedgewire. While the use of microscreen in a filter system was clearly contemplated, the microscreen septum of the present invention had not yet been evolved. As will be apparent, there are many advantages of the present microscreen septum over the wedgewire septum, not the least of which is the smaller microscreen aperture over the wedgewire slots which will pass particles of considerable length. In manufacture of the microscreen element, the desired aperture pattern is first produced through CAD design and transferred to a glass plate photo master. The pattern is then transferred in the form of a photosensitive resist to a stainless steel sheet. The photoetching process is completed by chemically machining or dissolving the apertures. Preferably the etching is conducted from both sides of the sheet at different rates to produce a tapered aperture. With the larger diameter at the inlet side and the smaller diameter at the exit side of the microscreen element, filter regeneration by a backwash process is greatly assisted to accomplish screen cleaning and filter cake release at the inlet side. The stainless steel microscreen sheet can have a thickness between 0.076 mm (0.003") and 0.381 mm (0.015") and will have a closely spaced aperture pattern with the aperture diameter being between 0.076 mm (0.003") and 0.381 mm (0.015") but never less than the sheet thickness. The actual microscreen element in the photograph of the filter septum in FIG. 5 has a thickness of 0.127 mm (0.005") with a hole diameter of 0.152 mm (0.006"). As seen in FIGS. 1-4 the microscreen element 12 has an inlet side 14 and an exit side 16 with apertures 18 tapering from a larger diameter at the inlet side to a smaller diameter at the exit side. With a nominal aperture size of 0.152 mm (0.006") diameter as shown in FIG. 5, the inlet side diameter can be approximately 0.229 mm (0.009") with the exit side being approximately 0.152 mm (0.006"). The supporting substrate or base plate 20 of the filter septum 10 will be substantially thicker than the microscreen sheet. In septum of FIG. 5 the base plate 20 is 3.175 mm (11 gage) stainless steel with 9.525 mm (3/8") diameter holes 22 on (12.7 mm) 1/2" centers, punched from exit side 24. Reinforcing support rods 26 are welded between adjacent rows of holes 22 on the exit side, and in the example of FIG. 5, the support rods 26 have a 3.175 mm (1/8") diameter and are on 101.6 mm 4" centers. If the final form of the filter septum is to be non-planar, for example a cylindrical drum, the base plate will be normally preformed to its final shape before brazing. The filter septum 12 is assembled to the base plate 20 by first spot welding the exit side 16 of the microscreen element 12 to the inlet side 34 of base plate 20 as can be seen in photograph FIG. 5. This is conveniently performed by laser or resistance welding. Final assembly of the microscreen element 12 to the base plate 20 is accomplished by furnace brazing so that all of the contact area has a layer of brazing alloy 28 between the exit side 16 of the microscreen element and the inlet side 34 of the base plate. In a preferred embodiment a nickel powder alloy is applied to the inlet side of microscreen element and fixed with an acrylic varnish carrier. The assembled microscreen element and base panel is loaded horizontally into a vacuum furnace and the temperature is ramped up to melt and wet the metal surface causing the brazing alloy to flow between the exit side of the microscreen element and the inlet side of the base plate permanently bonding the two together. Finally a thin fluorinated ethylene propylene coating 30 is applied to the inlet side of 14 of the microscreen element to assist in filter cake release and prevent clogging. according to the invention the coating is Teflon®, fluorinated ethylene propylene, and a single coating of Dupont 958-203 Teflon is sprayed on the surface and baked to assist the air cure.

Claims: A filter septum comprising a microscreen element (12) formed of a metal sheet having uniformly distributed apertures (18) through which a liquid to be filtered can be passed from an inlet side (14) to an exit side (16) of said element (12), and a metal base plate (20) thicker than said metal sheet and having an inlet side (34) and an exit side (24), said apertures (18) being formed in said element (12) by an electrochemical process to provide a taper to said apertures (18) from a larger diameter at the inlet side (14) of said element (12) to a smaller diameter at its exit side (16), said base plate (20) being provided with rows of uniformly distributed circular holes (22) perforated to have a diameter larger than the diameter of said apertures (18) at the inlet side (14) of said element (12), said element (12) being brazed to said base plate (20) so that a layer of brazing alloy (28) is provided between the exit side (16) of said element (12) and the inlet side (34) of said base plate (20), characterized in that a plurality of spaced parallel support rods (26) are welded to the exit side (24) of said base plate (20) between selected adjacent rows of said holes (22), a fluorinated ethylene propylene coating (30) being provided on said inlet side (14) of said element (12). A filter septum according to claim 1, wherein said brazing alloy (28) is a high nickel alloy. A filter septum according to claim 1 or 2, wherein said element (12) and said metal base plate (20) are made of stainless steel, said element (12) having a thickness comprised between 0.076 mm (0.003") and 0,381 mm (0.015"), said apertures (18) having a nominal diameter comprised between 0.076 mm (0.003") and 0,381 mm (0.015") but not less than the thickness of said element (12); said base plate (20) having a thickness of at least 8 times the thickness of said element (12), said holes (22) having a diameter of at least 10 times the diameter of said apertures (18). A method of producing a filter septum comprising the following steps: A) forming a microscreen element (12) by using a photo etching process to produce uniformly distributed apertures (18) in a metal sheet, said apertures (18) having a taper from a larger diameter at the inlet side (14) of said element (12) to a smaller diameter at the outlet side (16) of said element (12); B) producing a perforated metal base plate (20) having uniformly distributed circular holes (22) having a diameter larger than the diameter of said apertures (18) at the inlet side (14) of said element (12); C) brazing said element (12) to said base plate (20) so that a layer of brazing alloy (28) is provided between the exit side (16) of said element (12) and the inlet side (34) of said base plate (20); D) reinforcing said base plate (20) by welding a plurality of parallel support rods (26) to the exit side (24) of said base plate (20) between adjacent rows of said holes (22) at spaced intervals; and E) coating said inlet side (14) of said element (12) by spraying a layer of fluorinated ethylene propylene (30) on the inlet side (14) of said element (12) and baking the layer of fluorinated ethylene propylene (30) to assist air cure. A method according to claim 4, wherein said metal sheet in step A and said base plate (20) in step B are selected to be stainless steel, said metal sheet being selected to have a thickness comprised between 0.076 mm (0.003") and 0,381 mm (0.015"), and the nominal diameter of said apertures (18) being selected to be between 0.076 mm (0.003") and 0,381 mm (0.015") but not less than the thickness of said metal sheet. A method according to any claim 5, wherein said base plate (20) is selected to have a nominal 3.175 mm (11 gage) thickness and said holes (22) have a diameter of approximately 9.525 mm (3/8") on approximately 12.7 mm (1/2") staggered centers. A method according to any claim from 4 to 6, wherein in step A the exit side (16) of said element (12) is firstly welded to the inlet side (34) of said base plate (20) at a plurality of points; a powdered nickel alloy brazing material (28) being then applied to the exit side (16) of said element (12) and being fixed thereto with an acrylic varnish carrier, and wherein said brazing in step D is accomplished in a controlled atmosphere furnace. A method according to claim 7, wherein said brazing is accomplished in a vacuum furnace while said element (12) and said base plate (20) are held in a horizontal position and the temperature is increased allowing said powdered brazing material (28) to melt and wet the mutually contacting surfaces of said element (12) and of said base plate (20), thus brazing the exit side (16) of said element (12) to the inlet side (34) of said base plate (20) throughout all their mutually contacting surfaces.

Inventors: NEHLS BARRY L, NEHLS, BARRY L.
Applicants: HYDROFLOW INC, HYDROFLOW, INC.
Application year: 1994
Main classification: B01D39
IPC codes: B01D29